A selftightening drill chuck of this type has been described for example in German Open Patent Application No. 34 32 918. It allows both tightening of the chuck manually with the help of a chuck sleeve and selftightening of the chuck on a drill by the moment of reaction of the tool when the torque resistance of the tool causes a tightening motion on the chuck thread by the clamp jaws in the same way as the chuck sleeve.
Moreover in the known drill chuck the clamp jaws are supported or braced in the axial direction nonrotatably in a rotatable thrust piece mounted on the chuck body and the chuck sleeve is guided slidably axially on a chuck thread. The locking ring engaged on the chuck sleeve acts on the one hand to limit selftightening. The drill chuck because of the locking ring is not tightened so much during a drilling operation that it can no longer be loosened by hand. On the other hand the locking ring with the help of a spring force which acts on it in the rotational direction of the chuck sleeve corresponding to closing the drill chuck holds the clamp jaws by the chuck sleeve in a fixed unit on the tool shaft until the selftightening limit applies.
To maintain the engagement of the axially slidable chuck sleeve on the locking ring, a rotatable and axially unslidable coupling sleeve guided on the chuck body which on one side is couplable directly with the locking ring by a gear ring and on the other side is attached axially slidably and nonrotatably with the chuck sleeve. The locking ring is acted on by two compression springs, of which one is oriented in the axial direction between the locking ring and a circular shoulder of the drive spindle and the other is positioned in the circumferential direction between the locking ring and the chuck body in a groove extending over an arc segment in the locking ring. In toto this results in a comparatively expensive chuck structure.
Further a selftightening drill chuck without a selftightening or after-tightening limit is known in which the chuck body has a conical surface extending with straight outer surfaces parallel to the feed direction of the clamp jaws. A chuck thread in the form of a trapezoidal thread is located on this conical surface which engages with a transverse gear on each clamp jaw, which runs parallel to the feed direction of the clamp jaw. The chuck sleeve is held fixed from axial sliding on a circular collar of the chuck body by a roller body in its axial rear portion. The clamp jaws comprising essentially cylindrical bolts move or run in blind holes in the chuck sleeve in its rear sleeve portion. These blind holes must thus be drilled from the front in the chuck sleeve. They are located on a correspondingly small pitch circle because of their path inclined to the chuck axis and overlap mutually so that they can be made only with reduced precision. Not only the chuck precision but also the selftightening action of this known drill chuck is poor.